The present invention relates to an elevator having a safety controller to which several safety switch arrangements are connected. Usually the connections of the safety switch arrangements are connected to the safety controller as a safety chain with a series connection of the safety switch arrangements. Such safety chain is prescribed by common regulations as e.g. EN81-1. Different safety-related functions in an elevator, as e.g. the landing door closing function, the car door closing function, over-speed protection functions, etc. or control safety functions are provided with a corresponding safety switch arrangement whereby all the safety switch arrangements of the different safety functions are usually connected in series, although they might be connected directly to the safety controller at different ports. If any of these functions is not working according to regulations, e.g. if a landing door is still open, the safety switch of the safety switch arrangement is open and the safety controller to which the safety chain is connected gives an input to the elevator control to stop the elevator motor and to close the brakes (or to keep the in the locked state). The safety controller may also directly actuate the motor brake and disconnect the elevator motor.
As elevators are often operated over a very long time period, e.g. 30 years or more, the safety switch arrangements may become worn so that the connection or disconnection function of the safety switch arrangement does not work proper any longer. Therefore, it could happen that a safety switch arrangement has a short circuit in which case a corresponding landing door could stand open without the safety switch arrangement would indicate that status to the safety controller or interrupt the safety chain. The U.S. Pat. No. 7,980,363 discloses a safety arrangement where the safety switch arrangements comprise a resistor which is connected parallel to the safety switch of the corresponding safety switch arrangement. This solution reveals information about the number of open switches by measuring the resulting resistance value of the safety chain.
The US 2010/051391 discloses a method according to the present invention.